The present invention relates to a hydraulic striking mechanism including a cylinder, a percussion piston guided therein and a piston return device, with the percussion piston being charged on one side with a pressurized compressible driving medium (compressed gas). The piston return device is supported on the percussion piston so as to be displaceable independently of the percussion piston and returns the piston in a direction opposite to the direction of its operating or striking stroke. On its side facing the tip of the percussion piston, the piston return device is alternatingly connected to a pressure source for an incompressible driving medium (hydraulic oil) and to a pressure-free return conduit.
In spite of their otherwise differing operating modes, the prior art striking mechanisms have in common that the incompressible driving medium is directly coupled with the facing piston surfaces of the percussion piston. Movement of the percussion piston displaces the incompressible driving medium (i.e. the hydraulic oil) from the area in front of the one piston face and causes it to be replenished behind the other. The columns of fluid upstream and downstream of the two piston faces travel at the same speed as the percussion piston itself.
When the percussion piston hits the connected chisel or, for a direct strike, the material to be worked, the percussion piston is suddenly decelerated. The fluid column upstream of the piston face continues its movement in the direction o the strike; this produces the danger of cavitation. The sudden deceleration of the percussion piston and of the fluid column creates considerable pressure peaks at the opposite piston face. The direct coupling of the incompressible driving medium with the movement of the percussion piston is additionally a drawback in that the flow cross sections in the associated channels and in the control unit must be adapted to the greatest occurring piston velocity.
German Patent No. 2,941,443 [corresponding to U.S. Pat. No. 4,370,916] discloses a striking mechanism of the above-mentioned type which includes a piston return device that encloses the percussion piston in a spaced arrangement on the side facing away from the percussion piston tip and is in communication with piston rods on its side facing the percussion piston tip. The piston rods are components of cylinder units that are separate from the operating cylinder and serve to drive the piston return device in a direction opposite to the direction of the operating stroke of the percussion piston, that is, opposite to the striking direction, and thus return it. The fact that a resiliently supported latch is incorporated in the piston return device so as to be locked with the percussion piston makes it possible to take the percussion piston along in a direction opposite to the striking direction. During the operating stroke of the percussion piston in the striking direction, as soon as the latch is released from the percussion piston, the piston return device is set free and hurries after the percussion piston in the striking direction. The drawback of this prior art striking mechanism is that the coupling in and out of the piston return device, which is a mechanical action, is complicated from a manufacturing point of view and subject to malfunctions. Due to the relatively large masses of the combined interacting components, the piston return device permits striking rates only in an order of magnitude of no more than 60/min.